The prevalence of HIV infection is 28 to at least 50 times higher among people who inject drugs (PWID) compared to the general population. In North America, there were an estimated 267,000 persons living with HIV infection (PLHs) among 2 million PWIDs in 2012. Opioids represent the dominant class of injected agent, and in 2013 517,000 adults reported heroin use within the past year, representing an approximately 150% increase compared to 2007. Medication-assisted treatment (MAT) for opiate addiction, combined with needle and syringe exchange programs (NESP) have substantially reduced the risk of HIV transmission in PWIDs. Moreover, MAT reduces mortality among HIV-positive PWIDs (which is otherwise 3-fold higher than in PWIDs who are HIV-negative). MAT is predominantly available in the form of opioid agonist treatment with methadone or buprenorphine, with emerging use of opioid antagonist treatments (e.g. extended-release naltrexone). However, there are no recommendations currently available to guide the selection of MAT agent. Moreover, despite substantial evidence for immunomodulatory effects of opioids on immune responses, to our knowledge no studies have employed systems biology methods to evaluate MAT agents for their effects on parameters such as chronic inflammation?particularly important for HIV disease progression and present even in elite controllers or individuals who have achieved virologic control without antiretroviral therapy (ART). To address these questions, we have assembled an interdisciplinary group of investigators with expertise in HIV disease, the epidemiology of injection drug use and MAT intervention studies for PWID, innate immunity and inflammation, and computational biology. Using methods familiar to our research groups, we will carry out a prospective, longitudinal study of HIV-positive (on ART) vs. HIV-negative PWIDs starting MAT, recruited from the largest drug treatment center in New Haven, Connecticut. We will obtain samples of blood at baseline (day 0), and at day 3, 14, and at 1, 3, and 6 months after the start of MAT. Freshly collected samples of whole blood will be analyzed using multidimensional mass cytometry to evaluate the activation and differentiation state of cells of the innate and adaptive immune system, including novel analyses of platelet activation and function as an understudied contributor to chronic inflammation. In addition, we will carry out an unbiased analysis of innate immune pattern recognition receptor (PRR) function in monocytes and dendritic cells to assess potential alterations in PRR function that may contribute to dysregulated inflammatory responses. RNA-seq analyses of whole blood, together with metabolomic analyses of serum samples, will also be performed at these timepoints. We believe the integration of these cellular, immunologic, transcriptomic and metabolomic signatures will advance the biology of MAT in the context of HIV infection. Moreover, our design of a prospective study and the analysis of freshly isolated cells will facilitate insights that cannot be achieved with cryopreserved samples and should provide information to guide the selection of MAT agents.